Advanced water reactor technology

I. What are water reactors?

Water Reactors are nuclear power plants
that use water to control and remove the heat from the nuclear fuel in
order to convert heat to electricity. The first nuclear.power plant operated
by a utility company to produce electricity on a commercial grid was a
water- cooled reactor started up in 1957 at Shippingport, Pennsylvania,
in the United States. Since that time, over 330 water-cooled reactors
have been built and have produced electricity throughout the world. Water-cooled
reactors comprise over 80 per cent of all the world's nuclear power plants;
the remainder are gas-cooled reactors, liquid metal-cooled reactors or
graphite-moderated water-cooled reactors.

There are two basic types of water
reactors: those cooled by ordinary water, called the light water type,
and the heavy water type. The majority of the world's water reactors are
Light Water Reactors (LWRs), including both Pressurized Water Reactors
(PWRs) and Boiling Water Reactors (BWRs). Heavy water reactors are discussed
in Section X.

II. Why are improved water reactors desirable?

It is typical of any technology that,
as experience is gained, opportunities arise for improving the performance
and economics of the technology. Such is the case for water reactors,
which have a very large experience base in most industrialised countries.
Most of these countries reason that, if they are to continue to utilise
nuclear power to maintain national energy security in the future, they
should take advantage of the experience already gained to further improve
future water reactors. Hence, many of the industrial countries have extensive
programmes under way to advance the technology of water reactors.

III. What are the objectives of advanced
water reactor development?

Most advanced water reactor development
programmes have the dual aim of improving safety and reducing environmental
impacts while at the same time providing further reductions In the cost
of generating electricity. Environmental and public health benefits are
to be achieved through improvements in reactor design and operation to
further reduce the already low probability and potential consequences
of accidents -the so-called "residual risk" of reactor operations.

While it may appear to some that these
goals conflict with each other, in most cases, they do not. The objectives
of improved safety, reliability and economics being sought in advanced
water reactors can be, by and large, complementary. The same enhancements
that lead to improved reliability and economics--for example, simplified
system design and improved ruggedness of structures and components--may
also lead to improved safety.

IV. Can we improve existing operating water
reactors?

Many of the lessons learned from the
past 30 years of water reactor operation have been and continue to be
applied to the design and operation of existing water reactors. Indeed
many of the advances in technology being considered for future plants
came about because of this direct experience. Examples are the lessons
learned from the 1979 accident at Three Mile Island (TMI). Major improvements,
such as in the design of control rooms and instrument systems, have been
made in existing plants as a result of lessons learned at TMI and further
improvements are being made in designs for future plants. In addition,
several systems have recently been established among all countries which
operate nuclear power plants to share operational experience in an effort
to enhance the safety and performance of existing reactors. These include
the Incident Reporting Systems of the NEA and the International Atomic
Energy Agency (IAEA), as well as the recently organised World Association
of Nuclear Operators (WANO).

V. What changes are being made reactors?

Many different approaches are being
taken to improve future water reactors. In some cases, incremental changes
in design are being incorporated in plants now being built. These changes
include improvements to make plants easier and more economical to operate
and maintain, changes to plant safety systems to reduce the already low
residual risk of reactor operation, and changes to the fuel design to,increase
the efficiency of fuel utilisation. In other cases, designs are being
changed to reduce the complexity of the plant and to improve its safety.
An example is the new design for a containment structure which can be
cooled by natural circulation only. Such programmes will take a number
of years to develop and, hence, are not yet ready for construction commitments.
In still other cases, more revolutionary designs are being developed which
may require the operation of a prototype plant to test the design prior
to commercial commitment.

These approaches are exemplified in
the following projects now being carried out in some of the OECD countries:

A.
The French advanced PWR, the N4, is under construction in Chooz, France.
This 1400 MWe project aims at reducing the cost of nuclear electric power
through use of advanced components such as pumps, heat exchangers and
turbines which lower the capital cost and increase the efficiency of operation.
The safety of plant operation is enhanced through improvements in the
man-machine interface and other features.

The
N4 Advanced PWR at Chooz, France

Other countries have similar near-term
advanced water reactor projects. For example, three large advanced PWRs
called Convoy were recently completed in Germany. The United Kingdom's
first commercial water reactor, the Sizewell B PWR, is now under construction
with startup scheduled in 1994. A large ad- vanced BWR, jointly designed
by Japanese and U.S. firms, is under construction In Japan.

B.
Two mid-sized (about 600 MWe) advanced LWRs are under development in the
United States, with the major focus on plant simplification. For example,
the advanced PWR has 32 per cent fewer valves, 35 per cent fewer pumps,
and 45 per cent less pipe than a traditional PWR of comparable rating.
These simplifications are expected to greatly enhance the safety and reliability
of plant operation. Significant reductions in the cost and schedule of
plant construction are expected both from plant simplification and from
the application of modular construction techniques and a greater scope
for factory assembly. Major emphasis is also placed on passive safety
features which put less reliance on human intervention for accident management.
For example, emergency core cooling systems will not rely on pumping systems
requiring diesel generated electric power, and containments can be cooled
using natural rather than forced circulation. Such a passive cooled containment
system (PCCS) is shown below:

A mid-sized BWR being developed in
the U.S. is expected to include similar improvements in simplicity, safety
and economics. Several OECD Member countries, including Italy, Japan,
France and the Netherlands, are participating with the U.S. Electric Power
Research Institute in various aspects of the mid-sized advanced LWR development
programme.

C.
The PIUS (Process Inherent Ultimate Safety) reactor is under development
in Sweden. Because this reactor system has marked departures from existing
water reactor systems in areas such as reactivity control and primary
coolant system configuration, a largescale prototype system (or demonstration
plant) should probably be constructed to confirm the reliability of the
system. A primary design goal of this system (and a similar system in
Japan, the Intrinsically Safe Economical Reactor (ISER), is enhanced protection
of the core during postulated accidents. The goal is for core degradation
accidents to be prevented by passive means without reliance on the function
of active components and/or operator action following conceivable accidents.

VI. How long will it take to secure the
benefits of advanced water reactors?

In addition to the state of technical
development, the schedule for deployment of these advanced water reactor
systems depends in many cases on evolving government policies. In many
countries, ample resources and capabilities are now available for investment
in advanced water reactor deployment, provided national governments take
a leadership role in creating a stable regulatory climate to provide the
conditions needed for private investment. Various forms of international
co-operation such as exchanges of information and joint projects could
also make a valuable contribution to the development of advanced reactors.

VII. What about inherently safe reactors?

Improvements already made to existing
reactors and planned for future water reactors have and will continue
to achieve significant reductions in the residual risk of reactor operations.
Many of these features rely on passive safety features, or inherently
safe choricteristics. That is, they require less reliance on human participation
and intervention to ensure the reactor is always in a safe condition.

However, it is not technically sound
to claim that any reactor, be it a water reactor or some other type, is
an "inherently safe reactor". The use of this term is a misnomer and is
inappropriate for any power generating technology. All energy technologies
pose some risk, and it is necessary to evaluate the risks and benefits
of each technology carefully and objectively before reaching decisions
on new applications.

VIII. Are small reactors safer?

Two smaller reactors producing 500
megawatts of electricity each are not necessarily safer than one large
reactor producing 1 000 megawatts. Although some features which can be
provided in smaller reactors place greater reliance on more passive safety
features compared to existing larger reactors, many other factors which
affect overall safety are either not a function of size or favour large
reactors over smaller ones. therefore, electric supply organisations and
their national governments will continue to select the unit size of new
water reactors based on utility-specific factors such as grid size, economics,
and energy demand growth.

IX. Will advanced water reactors be less
costly?

The cost of nuclear power compared
to alternatives varies considerably with the age of the plant and from
country to country. In many cases, nuclear power is more economic than
electricity generated by coal or oil. In other cases, where lengthy construction
schedules and resulting high costs were incurred, or where coal, oil or
gas are currently available at low cost, nuclear power is more expensive.

One of the major objectives of the
advanced water reactor programmes in most countries is to further improve
the economics of nuclear power. Features being included in all advanced
water reactors focus on simplifying the design, using modular construction,
and other steps to reduce the initial capital cost. In some countries,
a coherent energy policy and the introduction of a streamlined and stable
licensing system is a critical prerequisite to such cost reduction efforts.
These factors, together with the anticipated increasing costs of available
fossil fuel alternatives, such as the cost of new pollution control systems,
are expected to enhance the economics of nuclear power in future years.

X. Heavy water reactors

The heavy water reactor uses a molecular
variation of ordinary water comprised of two atoms of deuterium for every
atom of oxygen, instead of the usual two atoms of hydrogen for every atom
of oxygen. A deuterium atom is twice as heavy as an ordinary hydrogen
atom. Heavy water has different nuclear properties than ordinary light
water, although its appearance and chemical behaviour are the same.

Commercial heavy water reactors were
pioneered by the Canadians and are often called CANDUs (for Canadian Deuterium
reactors). Among the advanced heavy water reactors being studied are two
which have been developed by Canada: the CANDU 3 with a rating of 450
MWe and the CANDU 6 Mark II with a 600 MWe rating.

Canada is also developing a larger
advanced heavy water reactor (the CANDU 6 Mark III) which will have a
rating of 700 MWe to 1150 MWe and will feature major advances in safety,
reliability and economics. The design programme began in mid-1987 and
is expected to produce a design for project commitment in the early 1990s.

An Advanced Thermal Reactor, a 606
MWe heavy water moderated BWR, is under development in Japan, with plans
for commercial operation in the late 1990s. This project is intended to
demonstrate major improvements in fuel utilisation and is viewed by Japan
as a possible transition to the ultimate Introduction of fast breeder
reactors.

XI. Role of the Nuclear Energy Agency

Development and deployment of advanced
water reactors should be the responsibility of the individual Member countries.
To share the costs and risks of such development, a number of countries
are already co-operating in the development of specific advanced water
reactor systems. In view of wider interest in the potential benefits of
these programmes, the NEA will examine the various positions on development
and deployment of advanced water reactors in Member countries.

REFERENCES

A Study on Advanced Water-Cooled Reactor Technologies,
a Report by an Expert Group, NEA (to be published in 1989).

Status of Advanced Technology and Design for
Water Cooled Reactors, Light Water Reactors, IAEA TECDOC-479, 1988 (a
companion document on heavy water reactors is under preparation).